1. Field of the Invention
The present invention relates to an induction heating apparatus, and more particularly, to the structure of an induction heating coil of an induction heating apparatus for continuously heating a conductive object.
2. Description of the Prior Art
One example of a continuous induction heating apparatus for heating a plated steel sheet in the prior art is shown in FIGS. 5 and 6. As shown in FIG. 5, a plated steel sheet 1 is continuously carried into an induction heating apparatus 3 as guided by guide members 2. Within the induction heating apparatus 3 is a solenoid type of induction heating coil 4 as shown in FIG. 6. The plated steel sheet 1 is conveyed as surrounded helically by the induction heating coil 4, i.e. passes through a central space of the induction heating coil 4. The induction heating coil 4 has a heat-insulating dielectric material at its outermost layer, and by passing an electric current through the induction heating coil 4, the plated steel sheet can be inductively heated.
The amount of electric power to be fed to the induction heating coil 4 depends upon the extent of a temperature rise and a production rate (a steel sheet conveying speed) required for the plated steel sheet 1.
Since the plated steel sheet 1 is to be heated while passing through the space at central portion of the induction heating coil 4, when a leading end portion of the plated steel sheet 1 is to be introduced into the heating apparatus 3, a leading wire is connected to the leading end of the plated steel sheet, then this leading wire is first fed through the induction heating coil 4, and subsequently the plated steel sheet 1 is led by the wire through the induction heating coil 4.
The heating apparatus 3 in the prior art shown in FIGS. 5 and 6 has the following shortcomings.
(1) Because of the fact that a solenoid type of induction heating coil 4 is employed, the object to be heated must be led into the apparatus by a leading wire. Therefore, labor is necessary for initially leading the plated steel sheet 1 into or from the heating apparatus 3.
(2) Since the induction heating coil 4 has a closed structure (spiral shape), heat-insulating dielectric material on the inner surface of the coil (on the surface facing the plated steel sheet 1) is impossible to maintain and inspect.
An induction heating apparatus obviating the above-mentioned shortcomings (1) and (2) has been developed. One example of such an apparatus will be explained with reference to FIG. 7. In this heating apparatus 10, while a solenoid type of coil 11 is employed, conductors 11a forming a part of the coil 11 are separable from the remainder of the coil. The conductors 11a can be opened and closed by means of a revolving mechanism, and at the time of closure, the opposite ends of the conductors 11a are connected via knife-edge type of contacts 12 to the other portions of the coil 11 to form a loop. When the loop is formed, electrical power is fed from a power supply section 13 to the coil 11, and an object to be heated is passed through a space at the central portion of the coil 11, whereby induction heating can be effected. It is to be noted that the contact 12 is made to have a sufficient connector capacity for allowing passage of a high-frequency current through the coil 11 without any trouble.
In this heating apparatus 10, before an object to be heated is made to pass through the apparatus for the first time, the conductors 11a are opened up to the positions depicted by dotted lines in FIG. 7. Then the heating apparatus 10 is moved while the object to be heated is kept stationary, and the object to be heated is set in the space at the central portion of the coil. In addition, if the conductors 11a are kept opened, the inner surface side of the coil 11 can be easily inspected.
However, the heating apparatus shown in FIG. 7 has the following shortcomings.
(1) Since a high current (1000-2000A) at a high frequency (about 10 KHz) flows through the contacts 12, the contact surfaces when opening and closing the conductors are likely to become rough. In addition, because twice as many contacts 12 as the number of turns of the coil 11 are necessary, a miscentering may occur when a plurality of the contacts 12 open and close. For such reasons, reliability would be lowered if it is used for a long period.
(2) Because contacts 12 and a mechanism for opening and closing the conductors 11a are necessary, the equipment cost is high.